A. Field of the Invention
The present invention relates to a method and device for generating a probing signal for use in a pulse code modulation (PCM) data communication system. The communication system of particular interest herein uses the public digital telephone network (DTN) to transmit data. The presence of Robbed-Bit-Signaling (RBS) and/or a Network Digital Attenuators (NDA), and codec conversion distortion within the DTN impacts negatively upon the communication system performance. Determining its presence in the system allows the communication devices to minimize the impact of the distortion. The probing signal of the present invention exhibits properties particularly well suited for distortion detection.
B. Description of the Related Art
For many years the public digital telephone network (DTN) has been used for data transmission between modems. Typically, a modulated carrier is sent over a local loop to a service provider (e.g., a Regional Bell Operating Company), whereupon the service provider quantizes the signal for transmission through the DTN. A service provider that is located near the receiving location converts the digital signal back to an analog signal for transmission over a local loop to the receiving modem. This system is limited in the maximum achievable data rate at least in part by the sampling rate of the quantizers, which is typically 8 kHz (which rate is also the corresponding channel transmission rate, or clock rate, of the DTN).
Furthermore, the analog-to-digital (A/D) and digital-to-analog (D/A) conversions are typically performed in accordance with a non-linear quantizing rule. In North America, this conversion rule is known as .mu.-law. A similar non-linear sampling technique known as A-law is used in certain areas of the world such as Europe. The non-linear A/D and D/A conversion is generally performed by a codec (coder/decoder) device located at the interfaces between the DTN and local loops. Alternatively, these devices are referred to herein as a DAC (digital-to-analog converter) and an ADC (analog-to-digital converter).
It has been recognized that a data distribution system using the public telephone network can overcome certain aspects of the aforesaid limitations by providing a digital data source connected directly to the DTN, without an intervening codec. In such a system, the telephone network routes digital signals from the data source to a client's local subscriber loop without any intermediary analog facilities, such that the only analog portion of the link from the data source to the client is the client's local loop (plus the associated analog electronics at both ends of the loop). The only codec in the transmission path is the one at the DTN end of the client's subscriber loop.
FIG. 1 shows a block diagram of a data distribution system. The system includes a data source 10, or server, having a direct digital connection 30 to a digital telephone network (DTN) 20. A client 40 is connected to the DTN 20 by a subscriber loop 50 that is typically a two-wire, or twisted-pair, cable. The DTN routes digital signals from the data source 10 to the client's local subscriber loop 50 without any intermediary analog facilities such that the only analog portion of the link from the server 10 to the client 40 is the subscriber loop 50. The analog portion thus includes the channel characteristics of the subscriber loop 50 plus the associated analog electronics at both ends of the subscriber loop 50. The analog electronics are well known to those skilled in the art and typically include a subscriber line interface card at the central office that includes a codec, as well as circuitry used to generate and interpret call progress signals (ring voltage, on-hook and off-hook detection, etc.). In the system of FIG. 1, the only codec in the transmission path from the server 10 to the client 40 is a DAC located at the DTN 20 end of the subscriber loop 50. It is understood that the client-side, or subscriberside, equipment may incorporate an ADC and DAC for its internal signal processing, as is typical of present day modem devices. For the reverse channel, the only ADC converter in the path from the client 40 to the server 10 is also at the DTN 20 end of the subscriber loop 50.
In the system of FIG. 1, the server 10, having direct digital access to the DTN 20 may be a single computer, or may include a communications hub that provides digital access to a number of computers or processing units. Such a hub/server is disclosed in U.S. Pat. No. 5,528,595, issued Jun. 18, 1996, the contents of which are incorporated herein by reference. Another hub/server configuration is disclosed in U.S. Pat. No. 5,577,105, issued Nov. 19, 1996, the contents of which are also incorporated herein by reference.
In the system shown in FIG. 1, digital data can be input to the DTN 20 as 8-bit bytes (octets) at the 8 kHz clock rate of the DTN. This is commonly referred to as a DS-0 signal format. At the interface between the DTN 20 and the subscriber loop 50, the DTN 20 codec converts each byte to one of 255 analog voltage levels (two different octets each represent 0 volts) that are sent over the subscriber loop 50 and received by a decoder at the client's location. The last leg of this system, i.e., the local loop 50 from the network codec to the client 40, may be viewed as a type of baseband data transmission system because no carrier is being modulated in the transmission of the data. The baseband signal set contains the positive and negative voltage pulses output by the codec in response to the binary octets sent over the DTN. The client 40, as shown in FIG. 1, may be referred to herein as a PCM modem.
FIG. 3 shows a .mu.-law to linear conversion graph for one-half of the .mu.-law codeword set used by the DTN 20 codec. As shown in FIG. 3, the analog voltages (shown as decimal equivalents of linear codewords having 16 bits) corresponding to the quantization levels are non-uniformly spaced and follow a generally logarithmic curve. In other words, the increment in the analog voltage level produced from one codeword to the next is not linear, but depends on the mapping as shown in FIG. 3. Note that the vertical scale of FIG. 3 is calibrated in integers from 0 to 32,124. These numbers correspond to a linear 16-bit A/D converter. As is known to those of ordinary skill in the art, the sixteenth bit is a sign bit which provides integers from 0 to -32124 which correspond to octets from 0 to 127, not shown in FIG. 3. Thus FIG. 3 can be viewed as a conversion between the logarithmic binary data and the corresponding linear 16-bit binary data. It can also be seen in FIG. 3 that the logarithmic function of the standard conversion format is approximated by a series of 8 linear segments.
The conversion from octet to analog voltage (or a digital representation of the analog voltage, as discussed above) is well known, and as stated above, is based on a system called .mu.-law coding in North America and A-law coding in Europe.
Theoretically, there are 256 points represented by the 256 possible octets, or .mu.-law codewords. The format of the .mu.-law codewords is shown in FIG. 2, where the most significant bit b.sub.7 indicates the sign, the three bits b.sub.6 -b.sub.4 represent the linear segment, and the four bits, b.sub.0 -b.sub.3 indicate the step along the particular linear segment. These points are symmetric about zero; i.e., there are 128 positive and 128 negative levels, including two encodings of zero. Since there are 254 non-zero points, the maximum number of bits that can be sent per signaling interval (symbol) is just under 8 bits. A .mu.-law or A-law codeword may be referred to herein as a PCM codeword. It is actually the PCM codeword that results in the DTN 20 codec to output a particular analog voltage. The codeword and the corresponding voltage may be referred to herein as "points."
Other factors, such as robbed-bit signaling, digital attenuation (pads), channel distortion and noise introduced by the subscriber loop, and the crowding of points at the smaller voltage amplitudes and the associated difficulty in distinguishing between them at the decoder/receiver, may reduce the maximum attainable bit rate. Robbed Bit Signaling (RBS) involves the periodic use of the least significant bit (LSB) of the PCM codeword by the DTN 20 to convey control information. Usually the robbed bit is replaced with a logical `1` before transmission to the client 40. The DTN performs robbed-bit signalling on a cyclic basis, robbing the lsb of an individual channel every sixth PCM codeword. In addition, due to the fact that a channel might traverse several digital networks before arriving at the terminus of the DTN 20, more than one PCM codeword per 6 time slots could have a bit robbed by each network, with each network link robbing a different lsb.
To control power levels, some networks impose digital attenuators that act on the PCM codewords to convert them to smaller values. Unlike most analog attenuators, a network digital attenuator (NDA) is not linear. Because there are a finite number of digital levels to choose from, the NDA will be unable to convert each codeword to a unique, lower magnitude codeword. This causes distortion of the analog level ultimately transmitted by the codec over the subscriber loop 50. RBS and an NDA can coexist in many combinations. For example, a PCM interval could have a robbed bit of type `1`, followed by an NDA followed by another robbed bit of type `1`. This could happen to a byte if a channel goes through a bit-robbed link, then through an NDA, then another bit-robbed link before reaching the DTN 20 codec.
It is evident that the above-described data transmission system may corrupt and distort the points that are used to transmit data through the system: e.g., lsbs may be robbed during some time slots thereby making some points unavailable in that time slot; digital attenuators may make some points ambiguous; the codec may not generate the analog voltages accurately; and, noise on the local loop may prevent the use of closely spaced points for a desired error rate. Thus it is desirable to be able to determine what types of distortion are present in the system so that the communication devices may minimize the effects of the distortion.